(a) Field of the Invention
The present invention relates to a liquid crystal display having an electrostatic discharge (ESD) protection circuit and a method for testing display quality using the circuit.
(b) Description of the Related Art
A liquid crystal display (LCD), uses the property of varying light transmittance according to the level of voltage applied to the liquid crystal to display images. The LCD has the advantage of requiring a lower voltage than that required by other types of displays.
Generally, the LCD is manufactured on a glass substrate. Because such a glass substrate is non-conductive, abruptly generated charges cannot spread over the glass substrate, making the LCD susceptible to damage from electrostatic charges. Consequently, it is possible for an insulating layer and other elements formed on the glass substrate to become damaged, from electrostatic charges.
To solve the above problem, it is common to use shorting bars on the glass substrate to connect all metal lines such as gate lines and data lines transmitting scanning signals and image signals to the pixels. Alternatively, a circuit, which employs nonlinear elements, to protect against electrostatic charges is used.
When shorting bars are connected to the metal lines, metal typically remains exposed around edges of the glass after a scribing process. In order to insulate the exposed metal, a non-conductive adhesive is thinly applied around the edges of the glass. Unfortunately, it is difficult to control the uniformity of the adhesive around the edges of the glass. Furthermore, this is an additional step that needs to be performed.
As stated before, a circuit is often used to protect against static electricity, and will be described hereinafter with reference to FIG. 1 and FIG. 2.
FIG. 1 shows a wiring diagram of a conventional LCD substrate using shorting bars. A plurality of gate lines 20 and data lines 30 are respectively formed horizontally and vertically on a substrate 10 to protect the LCD against static electricity. The gate lines 20 and data lines 30 are connected at the edge of the substrate 10 by shorting bars 40 and 41, respectively. The shorting bar 40 connecting the data lines 30 and the shorting bar 41 connecting the gate lines 20 are interconnected. A testing pad 60, for inputting test signals, is connected to the end of the shorting bar 40.
During the manufacture of the LCD damage caused by static electricity generated in the data lines 30 and the gate lines 20 is minimized by grounding the shorting bar 40, through which the static electricity is dispersed. The shorting bar 40 can also be used for detecting display defects in the LCD. That is, when applying a predetermined signal to the shorting bars 40 and 41, all pixels of the LCD simultaneously turn ON. However, if there are any defect in the data line 30, the gate line 20 or a thin-film transistor, the pixels appear black as no signals are transmitted thereto.
To effectively discharge static electricity, protection circuit 50 including diodes for protecting against static electricity can be additionally provided on one or both sides of each data line 30 and each gate line 20.
FIG. 2 shows a schematic diagram of a protection circuit shown in FIG. 1. The connection and grounding of the protection diodes are shown in detail in the drawing. Diodes D1 and D2 are connected to each gate line 20 and data line 30 in forward and reverse directions, respectively. All the diodes 50 are connected through a shorting line 51. Therefore, the diodes D1 and D2, the data shorting bar 40 and the gate shorting bar 41 are all connected together.
In such an LCD, when static electricity is generated in one of the data lines 30, the diode D1 connected to the data line 30 turns on and the electric charges move to the grounding line 51 via the diode D1. The electric charges pass through the shorting line 51, turns on the diode D2 of an adjacent data line 30, then moves through this adjacent data line 30. Static electricity is thus gradually dissipated by moving through adjacent lines in this manner.
Testing of display quality of the substrate is conducted in the same way as described above. After testing display quality, the shorting bars 40 and 41 formed at the edge of substrate are removed by grinding along a cutting line C/L. However, in the case where an IC is a COG (chip on glass) mode, formed on the substrate 10, since the shorting bars 40 and 41 are formed on a side opposite to the IC, the shorting bars 40 and 41 must be removed by cutting rather than grinding, thereby complicating the overall manufacturing process.
Furthermore, when an electrostatic discharge protection circuit is used, as all data lines are applied with the same voltage, it is not possible to detect a short in the data and gate lines. Accordingly, the only way to detect a short-circuit in the data and gate lines is to actually drive the panel and test for defects. This added step increases the manufacturing time, and consequently increasing the overall manufacturing costs.